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| author | Linus Torvalds <torvalds@linux-foundation.org> | 2017-05-02 10:21:17 -0700 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2017-05-02 10:21:17 -0700 |
| commit | c58d4055c054fc6dc72f1be8bc71bd6fff209e48 (patch) | |
| tree | 56527e28fc65d74d6d5e8397c502ccc87a9ec99b /Documentation/core-api/flexible-arrays.rst | |
| parent | ceb198bb007b84ead867e87a71ffe715c4412b15 (diff) | |
| parent | 9bb0e9cb04c82d6bf0e72f3207307d621083b801 (diff) | |
Merge tag 'docs-4.12' of git://git.lwn.net/linux
Pull documentation update from Jonathan Corbet:
"A reasonably busy cycle for documentation this time around. There is a
new guide for user-space API documents, rather sparsely populated at
the moment, but it's a start. Markus improved the infrastructure for
converting diagrams. Mauro has converted much of the USB documentation
over to RST. Plus the usual set of fixes, improvements, and tweaks.
There's a bit more than the usual amount of reaching out of
Documentation/ to fix comments elsewhere in the tree; I have acks for
those where I could get them"
* tag 'docs-4.12' of git://git.lwn.net/linux: (74 commits)
docs: Fix a couple typos
docs: Fix a spelling error in vfio-mediated-device.txt
docs: Fix a spelling error in ioctl-number.txt
MAINTAINERS: update file entry for HSI subsystem
Documentation: allow installing man pages to a user defined directory
Doc/PM: Sync with intel_powerclamp code behavior
zr364xx.rst: usb/devices is now at /sys/kernel/debug/
usb.rst: move documentation from proc_usb_info.txt to USB ReST book
convert philips.txt to ReST and add to media docs
docs-rst: usb: update old usbfs-related documentation
arm: Documentation: update a path name
docs: process/4.Coding.rst: Fix a couple of document refs
docs-rst: fix usb cross-references
usb: gadget.h: be consistent at kernel doc macros
usb: composite.h: fix two warnings when building docs
usb: get rid of some ReST doc build errors
usb.rst: get rid of some Sphinx errors
usb/URB.txt: convert to ReST and update it
usb/persist.txt: convert to ReST and add to driver-api book
usb/hotplug.txt: convert to ReST and add to driver-api book
...
Diffstat (limited to 'Documentation/core-api/flexible-arrays.rst')
| -rw-r--r-- | Documentation/core-api/flexible-arrays.rst | 130 |
1 files changed, 130 insertions, 0 deletions
diff --git a/Documentation/core-api/flexible-arrays.rst b/Documentation/core-api/flexible-arrays.rst new file mode 100644 index 000000000000..b6b85a1b518e --- /dev/null +++ b/Documentation/core-api/flexible-arrays.rst @@ -0,0 +1,130 @@ + +=================================== +Using flexible arrays in the kernel +=================================== + +Large contiguous memory allocations can be unreliable in the Linux kernel. +Kernel programmers will sometimes respond to this problem by allocating +pages with :c:func:`vmalloc()`. This solution not ideal, though. On 32-bit +systems, memory from vmalloc() must be mapped into a relatively small address +space; it's easy to run out. On SMP systems, the page table changes required +by vmalloc() allocations can require expensive cross-processor interrupts on +all CPUs. And, on all systems, use of space in the vmalloc() range increases +pressure on the translation lookaside buffer (TLB), reducing the performance +of the system. + +In many cases, the need for memory from vmalloc() can be eliminated by piecing +together an array from smaller parts; the flexible array library exists to make +this task easier. + +A flexible array holds an arbitrary (within limits) number of fixed-sized +objects, accessed via an integer index. Sparse arrays are handled +reasonably well. Only single-page allocations are made, so memory +allocation failures should be relatively rare. The down sides are that the +arrays cannot be indexed directly, individual object size cannot exceed the +system page size, and putting data into a flexible array requires a copy +operation. It's also worth noting that flexible arrays do no internal +locking at all; if concurrent access to an array is possible, then the +caller must arrange for appropriate mutual exclusion. + +The creation of a flexible array is done with :c:func:`flex_array_alloc()`:: + + #include <linux/flex_array.h> + + struct flex_array *flex_array_alloc(int element_size, + unsigned int total, + gfp_t flags); + +The individual object size is provided by ``element_size``, while total is the +maximum number of objects which can be stored in the array. The flags +argument is passed directly to the internal memory allocation calls. With +the current code, using flags to ask for high memory is likely to lead to +notably unpleasant side effects. + +It is also possible to define flexible arrays at compile time with:: + + DEFINE_FLEX_ARRAY(name, element_size, total); + +This macro will result in a definition of an array with the given name; the +element size and total will be checked for validity at compile time. + +Storing data into a flexible array is accomplished with a call to +:c:func:`flex_array_put()`:: + + int flex_array_put(struct flex_array *array, unsigned int element_nr, + void *src, gfp_t flags); + +This call will copy the data from src into the array, in the position +indicated by ``element_nr`` (which must be less than the maximum specified when +the array was created). If any memory allocations must be performed, flags +will be used. The return value is zero on success, a negative error code +otherwise. + +There might possibly be a need to store data into a flexible array while +running in some sort of atomic context; in this situation, sleeping in the +memory allocator would be a bad thing. That can be avoided by using +``GFP_ATOMIC`` for the flags value, but, often, there is a better way. The +trick is to ensure that any needed memory allocations are done before +entering atomic context, using :c:func:`flex_array_prealloc()`:: + + int flex_array_prealloc(struct flex_array *array, unsigned int start, + unsigned int nr_elements, gfp_t flags); + +This function will ensure that memory for the elements indexed in the range +defined by ``start`` and ``nr_elements`` has been allocated. Thereafter, a +``flex_array_put()`` call on an element in that range is guaranteed not to +block. + +Getting data back out of the array is done with :c:func:`flex_array_get()`:: + + void *flex_array_get(struct flex_array *fa, unsigned int element_nr); + +The return value is a pointer to the data element, or NULL if that +particular element has never been allocated. + +Note that it is possible to get back a valid pointer for an element which +has never been stored in the array. Memory for array elements is allocated +one page at a time; a single allocation could provide memory for several +adjacent elements. Flexible array elements are normally initialized to the +value ``FLEX_ARRAY_FREE`` (defined as 0x6c in <linux/poison.h>), so errors +involving that number probably result from use of unstored array entries. +Note that, if array elements are allocated with ``__GFP_ZERO``, they will be +initialized to zero and this poisoning will not happen. + +Individual elements in the array can be cleared with +:c:func:`flex_array_clear()`:: + + int flex_array_clear(struct flex_array *array, unsigned int element_nr); + +This function will set the given element to ``FLEX_ARRAY_FREE`` and return +zero. If storage for the indicated element is not allocated for the array, +``flex_array_clear()`` will return ``-EINVAL`` instead. Note that clearing an +element does not release the storage associated with it; to reduce the +allocated size of an array, call :c:func:`flex_array_shrink()`:: + + int flex_array_shrink(struct flex_array *array); + +The return value will be the number of pages of memory actually freed. +This function works by scanning the array for pages containing nothing but +``FLEX_ARRAY_FREE`` bytes, so (1) it can be expensive, and (2) it will not work +if the array's pages are allocated with ``__GFP_ZERO``. + +It is possible to remove all elements of an array with a call to +:c:func:`flex_array_free_parts()`:: + + void flex_array_free_parts(struct flex_array *array); + +This call frees all elements, but leaves the array itself in place. +Freeing the entire array is done with :c:func:`flex_array_free()`:: + + void flex_array_free(struct flex_array *array); + +As of this writing, there are no users of flexible arrays in the mainline +kernel. The functions described here are also not exported to modules; +that will probably be fixed when somebody comes up with a need for it. + + +Flexible array functions +------------------------ + +.. kernel-doc:: include/linux/flex_array.h |